The causal agent of the bacterial canker was Pseudomonas savastanoi pv. Fraxini, a particular pathovar which specifically infects ash trees. Infection typically enters through wounds or natural openings (lenticels) and is favoured by cool, wet conditions. The disease weakens trees, making them more vulnerable to other pathogens and environmental stress – potentially encircling stems and causing the tree to die.
New infections are usually on younger trees and branches around ten years old. The infection manifests as red-brown, lens-shaped blisters on the bark of young trees, especially at wounds, and in springtime or early summer these young cankers can seep a yellowish slime.
The disease progression is slow and can persist for many years, with blisters cracking open, revealing brown-black, necrotic tissue leading to the formation of erumpent (Picture below) or sunken patches with roughened, dark, corky bark cankers on branches and stems.
These open cankers form as the bark dies and peels back and these cankers can potentially grow and gridle or encircle stems, causing branch or tree death in younger trees.
Infected trees tend to bear many separate infections and become disfigured (see Picture below). Long, vertical cracks sometimes over 1 metre in length, may develop on the trunk.
Severely infected younger trees may exhibit new epicormic growth from dormant buds on the trunk and infected trees can become more susceptible to other pests and diseases. While young trees may have severely restricted growth and may eventually die, the incidence of death in mature trees is fairly low.
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